1. Field of the Invention
The invention relates generally to measurement-while-drilling and logging-while-drilling tools and, more particularly, to arrangements for packaging of the sensor and detector portions of the tools.
2. Description of the Related Art
Measurement-while-drilling (MWD) and logging-while-drilling (LWD) devices are used to determine wellbore parameters and operating conditions during drilling of a well. These parameters and conditions may include formation density, gamma resistivity, acoustic porosity, and so forth. In a typical drilling run, only some of these parameters and conditions may be of interest, however. MWD and LWD tools generally include a sensor portion that contains the sensors of the type desired and a processor and associated storage medium for retaining the sensed information. Additionally, a telemetry system is often used to transmit the sensed information uphole. The telemetry system may include a mud pulser, acoustic telemetry option, or an electromagnetic transmission system.
The sensor portion of MWD or LWD systems is typically housed within a drill collar in a such a manner that the sensor portion cannot be easily removed and replaced. In fact, removal and replacement of the sensor portion typically requires that the drill string be removed from the wellbore, and then the portions above and below the drill collar housing the sensor portion be disassembled from the drill collar. This operation is time-consuming and, therefore, costly. Additionally, the drill collars involved are quite heavy and unwieldy and the process of changing out a sensor section runs the risk of damaging the components. Further, if some of the sensor components malfunction, the entire drill collar must often be removed and shipped off site for repair or replacement. Shipping tools back to a repair center is costly and time consuming.
There are several conventional methods for packaging sensor components within a drill collar. In one method, exemplified in U.S. Pat. No. 5,216,242, issued to Perry et al., sensors and detectors are hardwired within the drill collar sub and accessable via removable hatches. Another packaging arrangement is illustrated in U.S. Pat. No. 4,547,833, issued to Sharp. In this arrangement, the sensors and detectors are mounted upon a chassis, which is then retained centrally within an outer cylindrical housing. These components are then secured together with a number of fasteners and integrated into a drill string. Of course, to change out or repair the sensors and detectors, one must first remove the adjacent drill string components, as well as the various fasteners, and then remove the outer housing from the chassis. U.S. Pat. No. 5,613,561 issued to Moriarty illustrates a similar packaging scheme wherein components mounted on the chassis are accessible through ports.
MWD and LWD tools have high capital costs and operating costs. Indeed, the high costs associated with LWD tools have made them unattractive for use with land-based wells. Conventional packaging arrangements make it difficult and expensive to design for the three basic hole sizes (8½″, 9½″, and 12¼″). Traditional MWD/LWD tool design has required unique tool for each hole size. Each tool requires many man-years to design and develop. Also, field inventory must be kept on hand for every size, multiplying costs further. To overcome these difficulties, manufacturers often “orphan” one hole size, and adapt a tool from one of the other two hole sizes for the orphaned hole size. For example, a tool designed to be run into an 8½″ hole would be provided with an adapter and run into a 9½″ hole. Unfortunately, the quality of the log of data obtained in this manner is less than satisfactory. LWD tools, in particular, are designed for a particular hole size. The components are integral to the collar. When they are used in a hole size that they were not designed for, the measurement is either lost or seriously degraded. Some tools use a sleeve to improve the measurement by displacing mud away from the measurement sensors. This, however, has limited success because the sensors remain in their original location, yet are now even further displaced from the formation that they are trying to measure the properties of.
The present invention addresses the problems of the prior art.